Polybenzimidazole (PBI) is a polymeric material comprising very high levels of heat resistance, strength, and chemical stability and may be used in many applications, such as, heat-resistant fibers, coating varnishes, etc. However, polybenzimidazoles that currently are commercially available are expensive and have lack proton exchanging ability necessary for polyelectrolyte membrane (PEM) fuel cells. This significantly limits their use as components in electrochemical applications.
Sansone, et al., in U.S. Pat. Nos. 4,814,399 and 5,599,639, describe synthetic modification of PBI by removing imino hydrogens with an alkali metal hydride and reacting the resulting anion with a sulfone or a haloalkyl phosphonate to yield a PBI-N-alkane sulfonate or PBI-N-alkane phosphonate, respectively. The anion also may be reacted with an alkyl halide (U.S. Pat. No. 4,898,917) or an aryl fluoride (U.S. Pat. No. 4,933,397) to yield an N-alkyl PBI or an N-aryl PBI, respectively.
Savinell, et al., in U.S. Pat. No. 5,525,436, describe a technique of imbibing polyprotic inorganic acids, such as H2SO4 or H3PO4, into PBI films to yield acid proton conducting membranes. In U.S. Pat. No. 5,716,727, Savinell, et al. describe direct casting of an H3PO4 saturated PBI membrane from trifluoroacetic acid solution. Wei, et al., in U.S. Pat. No. 5,422,411, describe synthesizing sulfonic acid and phosphonic acid substituted α,β,β-trifluorostyrene polymers.
Fuel cells are electrochemical cells that generate electrical current by oxidation and reduction reactions that occur at electrodes known as a cathode and an anode, respectively. Between the two electrodes is an electrolyte for conducting ions generated at the electrodes. Fuel cells that use an ion exchange polymer as the electrolyte are referred to as polyelectrolyte membrane (PEM) fuel cells. Oxygen is a typical oxidant fuel while typical reductant fuels are hydrogen and methanol.
In the case of hydrogen/oxygen fuel cells where oxygen gas is the oxidant and hydrogen gas is the reductant, protons are transported through the membrane from the anode to the cathode. The electrolyte used must possess high proton conductivity as well as chemical inertness toward the reactants. High oxidative and reductive stability are thus an important requirement for hydrogen/oxygen fuel cell electrolytes. Since these cells produce water as a byproduct and may be operated at elevated temperatures, high thermal, as well as, hydrolytic stability in aqueous acidic media also are required. Generally, a good membrane for the PEM fuel cells has to meet the following criteria:
1) chemical and electrochemical stability in the fuel cell operating environment;
2) mechanical strength and stability under cell operating conditions;
3) high proton conductivity, low permeability to reactant gas, and high water transport; and
4) low production cost.
Cation exchange polymers based on poly 2,2′-(m-phenylene)-5,5′ bibenzimidazole (PBI) having the following structure: have been used in PEM fuel cell electrolytes and have inherent thermal and chemical stability resulting from the all aromatic character. This polybenzimidazole also possesses good mechanical strength and is commercially available from Celanese Corporation (Charlotte, N.C.). They are also reasonably tractable and can be dissolved in N,N-dimethyl acetamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone.
Other polyelectrolytes for PEM fuel cell use include Nafion® (Dupont; Wilmington, Del.) commercially available from a perfluorinated aliphatic polysulfonic acid and sulfonated polystyrene, described in U.S. Pat. Nos. 3,282,875 and 4,330,654. Nafion® possesses good proton conductivity and excellent chemical stability, but suffers from high cost and intractability. Additionally, sulfonated polystyrene lacks chemical and thermal stability and tends to limit its use in PEM fuel cell applications.
Therefore, there is a need for an inexpensive and efficient material, such as one that has high thermal and chemical stability, high ionic conductivity, miscibility with other polymers, and good mechanical strength and tractability, suitable for solid polymer electrolytes in electrochemical applications, and especially for high temperature polymer electrolyte membrane (PEM) fuel cells and in electrochemical applications.